Adult bone mass and density depend, in part, upon peak bone mass achieved by the end of the second decade of life and differences in peak bone mass may account for half the variability in bone mass in elderly persons. Low hip bone density is a strong predictor of hip fractures. Thus, the risk of fracture in the elderly is largely determined early in life. The major determinants of peak bone mass appear to be heredity, gender, age, adolescent hormonal status, calcium nutrition and physical activity. Of these, only nutrition and physical activity are modifiable determinants and our group has previously shown that calcium supplementation in adolescent women is not effective in achieving permanent gains in bone mass. The broad long term objective of the proposed research is to contribute to an understanding of the role of exercise in the acquisition of bone in adolescent girls. We will examine the effect of two exercise interventions involving either a small number of high force and high loading rate impacts or a large number of low force and low loading rate impacts to the lower extremities. A control group will perform exercises involving no additional lower extremity loading. The interventions designed on the basis of pilot studies and bone remodeling theories, will take less than 10 minutes to perform and will be conducted three times per week for a duration of two 36 week school years as part of the warm-up period in middle school physical education classes. Lower extremity loading throughout entire days will be directly measured using a computerized force monitor and accelerometer estimates of the number and magnitude of impacts will also be obtained. We hypothesize that ten minutes of high force/high loading rate exercise conducted three times a week for a period of 36 weeks will have a significant effect on bone gain in sedentary adolescent girls but will show little or no effect in active girls. It is also possible that ten minutes of low force/low loading rate exercise conducted for the same duration may have significant effect on bone gain in either sedentary or active adolescent girls. The load measurement results will also allow current theories of bone remodeling to be quantitatively tested in this important age group. A positive outcome from the proposed study could have a significant public health impact since the proposed intervention could be easily implemented in public schools nationwide.